1. Field of the Invention
The present invention relates to a trailing element device disposed within a headbox of a web forming apparatus. More particularly, the present invention relates to a trailing element device for dividing a flow of stock flowing through a slice chamber of the headbox towards a slice lip of the headbox.
2. Information Disclosure Statement
In a papermaking machine, paper stock or furnish is supplied to a headbox which defines a slice chamber and a slice lip such that the stock is ejected from the slice lip onto a moving fourdrinier wire. Water is drained through the fourdrinier wire so that a web is formed on the upper surface of the wire.
In the formation of multi-layer webs, a first or primary headbox has been used to deposit a first layer of stock onto a moving fourdrinier wire. Subsequently, a secondary headbox disposed downstream relative to the primary headbox deposits a second layer of stock onto the first layer such that the first layer is sandwiched between the second layer and the fourdrinier wire.
More recently, with the introduction of the "STRATAFLO" headbox, a plurality of furnishes are supplied to a single headbox. "STRATAFLO" is a common law mark of Beloit Corporation. Such furnishes are each supplied between adjacent trailing elements disposed within the slice chamber of the "STRATAFLO" headbox.
The trailing elements of the "STRATAFLO" headbox are hingedly secured to the upstream end of the slice chamber and extend in a direction from the upstream end of the slice chamber towards the slice lip. Each of the elements freely floats within the slice chamber for reducing turbulence of the stock within the slice chamber. By this means, the different stocks are simultaneously ejected from the headbox and interface immediately downstream of the downstream end of the respective trailing elements.
In the formation of multi-ply webs using a "STRATAFLO headbox, a relatively cheap furnish is supplied as the lower or intermediate layer, and such cheap furnish is covered with a finer, more printable top layer.
With the aforementioned arrangement, a problem has been experienced in that during passage of the respective stocks along the surfaces of each trailing element, fluid vortices tend to form adjacent to the surfaces of the trailing element. The axes of such vortices are usually oriented in the flow direction, that is, the machine direction. These fluid vortices cause problems in the production of a multi-layered paper product in that when the headbox is supplied with a different furnish on either side of the trailing element, these vortices tend to lead to mixing of the two layers in such a way as to create a streaky appearance in the resultant web.
More particularly, since these vortices are large enough to fill the outer layer of the stock jet, they can quickly sweep material from the interface between the layers to the outer surface of the jet. This leads to the deposit of center layer material on the outer surface of the resultant web.
As such vortices can be initiated by such things as the wake downstream from a perforated plate or a tube bank, or by something as simple as a change in direction of the flow, it is very difficult to completely eliminate them from the headbox. The effect of such vortices may be minimized by setting the web quickly after the flow leaves the divider. However, such setting or drying of the web close to the headbox is not always a practical approach to the problem in view of the geometry of the forming equipment.
The present invention seeks to overcome the aforementioned problem by reducing the strength of machine direction vortices present in the flow between trailing elements. This is accomplished by modifying the surface of the elements so as to provide surface resistance to flow in a cross-machine direction without increasing the resistance in the machine direction. The aforementioned cross-machine resistance is created by cutting grooves in the surface of the trailing element or by applying small bars to the sheets, such bars being oriented in a machine direction. The objective of the invention is to dissipate the rotational energy of the vortices and to trigger the formation of smaller vortices from the large ones. The smaller vortices should dissipate the rotational energy faster by their interaction on each other. The smaller vortices reduce the scale of mixing within each portion of the stock jet, thereby minimizing the tendency of central material to migrate to the surface.
Therefore, it is a primary object of the present invention to provide a trailing element device that overcomes the aforementioned problems associated with the prior art proposals and which makes a considerable contribution to the art of forming a web from stock.
Another object of the present invention is the provision of a trailing element in which at least one of the surfaces of the trailing element defines a plurality of parallel-spaced grooves which extend in a direction from the upstream to the downstream end of the sheet for inhibiting the formation of machine direction vortices within the flow of stock that would otherwise cause streaking of the resultant web.
Another object of the present invention is the provision of a trailing element device in which a first current of a first stock is disposed on and flows past a first surface of the element and a second current of a second stock flows past a second surface of the element, the arrangement being such that formation of machine direction vortices along the surfaces is inhibited by a first and second plurality of grooves so that when the first and second stocks interface downstream relative to the downstream end of the sheet, migration between the stocks is minimized, thereby reducing streaking in the resultant web.
Another object of the present invention is the provision of a first plurality of grooves defined by a first surface of the trailing element and a second plurality of grooves defined by a second surface of the trailing element, the first plurality of grooves being offset relative to the second plurality of grooves such that rigidity of the sheet in a cross-machine direction is maintained.
Another object of the present invention is the provision of a trailing element in which, in addition to reducing the tendency for machine direction streaks to occur, the break-up of larger scale vortices into smaller scale structures improves the small scale orientation of fibers in the sheet and promotes stronger internal bonding.
Other objects and advantages of the present invention will be readily apparent to those skilled in the art by a consideration of the detailed description contained hereinafter taken in conjunction with the annexed drawings.